1. Technical Field
The present invention relates to an image forming apparatus.
2. Related Art
An image forming apparatus for carrying out a color printing operation includes an apparatus of a tandem type which has, for each color, an image forming portion for transferring a toner image having each color onto a paper (which is constituted by a photosensitive member, an exposing device, and the like).
In the image forming apparatus of the tandem type, image forming portions for respective colors (Y (yellow), M (magenta), C (cyan) and K (black)) carry out a printing operation over a single paper, thereby forming toner images having the four colors. Consequently, a single color image is formed.
For this reason, when a transfer timing onto a transfer belt is deviated finely due to a change in a temperature of the image forming portion or a variation with time, there is caused a so-called registration shift in which only a specific color is printed with a shift in a single image.
As a registration adjusting technique for correcting the registration shift, the image forming portion for each color transfers a registration adjusting patch onto the transfer belt, an extent of a shift of the registration adjusting patch for each color on the transfer belt from a reference position is detected by a sensor, and a write timing of the image forming portion is changed to eliminate an amount of the registration shift.